Bit mapping for variable-size higher-order digital modulations

ABSTRACT

The invention is concerned with the mapping of m input bits to 2 m  modulation symbols of a two-dimensional symbol constellation. A quarter-quadrant constellation of 2 m-4  modulation symbols that are located in a first quadrant of the two-dimensional signal plane is formed with each modulation symbol associated with a respective m-4 bit label. A quarter constellation of the two-dimensional symbol constellation is formed by adding to the quarter-quadrant constellation three copies of the quarter-quadrant constellation rotated by −90 degrees, 180 degrees, and −270 degrees, respectively, and then displacing the quarter constellation by a shift value Δ, with each modulation symbol associated with a respective m-2 bit label. The two-dimensional symbol constellation is then formed by adding to the quarter constellation three copies of the quarter constellation rotated by +90 degrees, 180 degrees, and +270 degrees, respectively. Each symbol of the two dimensional constellation is associated with a respective m bit label of the m input bits.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

This invention relates generally to digital communications; and moreparticularly to higher order modulations used in high-speed digitalcommunication systems.

2. Description of Related Art

In digital communication systems, more than one digital modulationscheme may be employed for the transfer of digital information. Digitalmodulations are generally susceptible to noise and other signalimpairments introduced by the communication channel. In order to copewith different channel conditions, some high-speed digital communicationsystems use a multitude of digital modulations, e.g., QPSK, 8-QAM,16-QAM, 32-QAM, 64-QAM, 128-QAM, etc., and different levels of codingand transmit power. While one channel may support a lower ordermodulation, e.g., QPSK, permitting only a lower data rate, anotherchannel may support a higher order modulation, e.g., 64-QAM, providinghigher data rate. Commonly used coding schemes include forward errorcorrection (FEC) coding such as Reed-Solomon coding and inner channelcoding such as convolutional coding, trellis coded modulation, or TurboCoding, etc. Between FEC coding and inner coding the order of data isusually permuted by an interleaver.

Multiple digital modulations are employed in a variety of digitalcommunication systems, e.g., in voiceband modems, digital subscriberlinks (DSLs), cable modem systems, and a wide variety of wirelesssystems. The particular manner in which higher order constellations areconstructed and the way in which coding is employed and bits are mappedinto modulation symbols represents an important topic in designingcommunication systems.

Sometimes there exists a need to augment existing standards forcommunication systems with additional capabilities. For example, in theparticular case of downstream transmission in cable modem systems onemay want to add to existing 64 QAM and 256 QAM modes, for example.

SUMMARY OF THE INVENTION

A method according to the present invention maps m input bits to 2^(m)modulation symbols of a two-dimensional symbol constellation. Fortypically m≧6, the method consists in forming first a constellation of2^(m−4) modulation symbols suitably arranged in the first quadrant ofthe two-dimensional signal plane. This constellation is called aquarter-quadrant constellation and has one-sixteenth the size of thedesired two-dimensional symbol constellation. Each quarter-quadrantconstellation symbol is uniquely associated with a respective m-4 bitlabel of the m input bits.

A quarter constellation of the two-dimensional symbol constellation isthen formed by adding to the quarter-quadrant constellation three copiesof the quarter-quadrant constellation rotated by −90 degrees, 180degrees, and −270 degrees, respectively, and then displacing the quarterconstellation by a shift value Δ. Displacing the quarter constellationby the shift value Δ causes the symbols to coincide with symbols of thedesired two-dimensional symbol constellation. The quarter constellationhas one-fourth the size of the two-dimensional symbol constellation.Each symbol of the quarter constellation is then associated with arespective m-2 bit label of the m input bits, wherein m-4 bits of them-2 bit label are inherited from the quarter-quadrant constellation andtwo further bits of the m-2 bit label are used to distinguishquarter-quadrants of the quarter constellation.

The two-dimensional symbol constellation is then formed by adding to thequarter constellation three copies of the quarter constellation rotatedby +90 degrees, 180 degrees, and +270 degrees, respectively. Each symbolof the two-dimensional symbol constellation is then associated with anm-bit label, wherein m-2 bits of the m-bit label are inherited from thequarter constellation and two further bits of the m input bits are usedto distinguish quarters of the two-dimensional symbol constellation.

The mapping method of the embodiment supports various constellationsizes including 64 QAM, 128 QAM, 256-QAM, 512 QAM, and 1024 QAM. Thetechnique may be extended in to other constellations as well. The 64QAM, 256 QAM, and 1024 QAM constellations are formed as squareconstellations from quarter quadrant constellation, which are squareconstellations in the first quadrant. The 128 QAM and 512 QAMconstellations are obtained from initially rectangular quarter quadrantconstellations in the first quadrant, of which a plurality of symbolsare repositioned such that the 128 QAM and 512 QAM constellations becomecross constellations.

Other features and advantages of the present invention will becomeapparent from the following detailed description of the invention madewith reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a system diagram illustrating a Cable Modem (CM) communicationsystem constructed according to the present invention;

FIG. 2 is a block diagram illustrating the communication path of acommunication system that may operate according to the presentinvention;

FIG. 3 is a flow chart illustrating operation according to a firstembodiment of the present invention;

FIG. 4 is a block diagram illustrating an encoder constructed accordingto an embodiment of the present invention;

FIGS. 5A, 5B, 5C, and 6 are diagrams illustrating a generalized mappingof input bits to modulation symbols according to an embodiment of thepresent invention;

FIGS. 7A and 7B are diagrams illustrating aspects of the presentinvention as employed in forming a 256 quadrature amplitude modulated(QAM) constellation;

FIGS. 8A and 8B are diagrams illustrating aspects of the presentinvention when applied to a 512-QAM constellation;

FIGS. 9A and 9B are diagrams illustrating aspects of the presentinvention as applied to a 1024-QAM constellation;

FIG. 10 is a flow chart illustrating operation according to the presentinvention in creating input bits from information bits according to anembodiment of the present invention;

FIG. 11 is a block diagram illustrating an encoder that employs trellisencoding operations according to an aspect of the present invention; and

FIG. 12 is a diagram illustrating the manner in which forward errorcorrection frames may be constructed according to various embodiments ofthe present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 is a system diagram illustrating a Cable Modem (CM) communicationsystem 100 that is constructed according to/operating according to thepresent invention. The CM communication system 100 includes a pluralityof CMs (shown as CM #1 11, a CM #2 115, and a CM #n 121), a CableHeadend Transmitter 120, and a CMTS 130 (that may reside internal to, orexternal to the Cable Headend Transmitter 120). Each of the CMs 111-121,the Cable Headend Transmitter 120, and the CMTS 130 communicativelycouples to a CM network segment 199 that facilitates communicationsthere between according to the present invention. While the CM networksegment 199 is shown as a single entity, it is understood that the CMnetwork segment 199 includes a plurality of elements including cablemedia, routers, splitters, couplers, relays, and amplifiers, forexample, without departing from the scope and spirit of the invention.In the illustrated embodiment, the CM network segment 199 includes bothcoaxial cable and fiber media and is a Hybrid Fiber Coaxial (HFC)system. In some embodiments of the present invention, the CMcommunication system 100 operates consistently with one or more versionsof the Data Over Cable System Interface Specification (DOCSIS), andparticularly according to the ITU-T J.83B Annex to DOCSIS.

The CM network segment 199 supports communications between the CMs111-121, the Cable Headend Transmitter 120, and the CMTS 130. The CMTS130 may be located at a local office of a cable television company or atanother location within a CM communication system. The Cable HeadendTransmitter 120 provides a number of services including audioprogramming, video programming, local access channels, as well as anyother service known in the art of cable systems. Each of these servicesis provided to one or more users corresponding to CMs 111-121. However,as is known, cable subscribers may receive Cable Headend Transmitter 120service without receiving CMTS 130 service, and vice versa.

The service provider employs the CMTS 130 to provide network accessservices to the CMs 111-121, e.g., allows the users to access theInternet, Wide Area Networks, and other data services via the CMTS 130.The CMTS 130 provides many of the same functions provided by a digitalsubscriber line access multiplexer (DSLAM) within a digital subscriberline (DSL) system. The CMTS 130 receives incoming traffic in from theCMs 111-121 and routes it to an Internet Service Provider (ISP) forcoupling to the Internet, as shown via the Internet access. At the CMTS130, the CM service provider includes a number of resources forproviding third-party ISP access, accounting and logging purposes,dynamic host configuration protocol (DHCP) assignment andadministration, the storage of Internet protocol (IP) addresses for theCMs 111-121, and server control for the Data Over Cable ServiceInterface Specification (DOCSIS), a major standard used by U.S. cablesystems in providing Internet access to users.

Downstream information flows to the CMs 111-121 from the CMTS 130.Upstream information flows from the CMs 111-121 to the CMTS 130. In atypical installation, the CMTS 130 services as many as 1,000 CMs 111-121on a single 6 MHz bandwidth channel. Since a single channel having abandwidth of 6 MHz is capable of 30-40 megabits per second of totalthroughput, the CMTS 130 via the CM network segment 199 is capable ofserving the 1,000 CMs 111-121 with significantly better performance thanis available with standard dial-up modems or even ISDN service.

FIG. 2 is a block diagram illustrating the communication path of acommunication system that may operate according to the presentinvention. The communication path may be an upstream or downstream pathin the CM communication system 100 of FIG. 1. Alternately, thecommunication path may reside within another digital communicationsystem, e.g., satellite communication system, fixed wirelesscommunication system, cellular wireless communication system, wiredcommunication system, etc. The components illustrated in FIG. 2 are asubset of the components that are contained within a communicationdevice.

A forward error correction (FEC) coder 202 of a transmitter receivesinformation bits and codes the plurality of information bits to producea plurality of FEC blocks. The FEC coder 202 may employ Reed-Solomon oranother type of FEC coding. Interleaver 204 receives the plurality ofFEC blocks from FEC block 202 and interleaves the FEC blocks. Arandomizer 206 couples to the output of interleaver 204, receives theinterleaved FEC blocks, and randomizes the FEC blocks. In combination,the interleaver 204 and the randomizer 206 receive a plurality of FECblocks from FEC coder 202 and operate upon the FEC blocks to produce aplurality of interleaved and randomized FEC blocks. Frame synch trailer(FST) insert operator 208 appends frame synch trailers to the pluralityof interleaved and randomized FEC blocks to form an FEC frame.

Encoder 210 receives the FEC frame and maps the bits of the FEC frame tomodulation symbols according to an embodiment of the present invention.The encoder 210 produces a plurality of modulation symbols that aretransmitted via channel 212 to a decoder 214 located in a receiver. Theencoder 210 of FIG. 2 maps input bits of the FEC frame to modulationsymbols according to an embodiment of the present invention that will bedescribed further with reference to FIGS. 3-12. The modulation symbolsproduced by the encoder 210 may be converted in frequency prior to theirtransmission upon the channel 212. As will be illustrated further inFIGS. 4 and 11, the encoder 210 may use coding such as trellis coding,convolutional coding, Turbo Coding, or another type of channel coding.

Decoder 214 receives the plurality of symbols that have been operated onby channel 212 and decodes the plurality of symbols to produce a decodedoutput (according to the coding employed by encoder 210). The output ofdecoder 214 is received by a FST removal operator 216 that removes theframe synch trailer and/or otherwise operates upon the output of thedecoder 214. The output of the FST removal operator 216 is received byde-randomizer 218, which de-randomizes and then by de-interleaver 220that de-interleaves the received data. The output of de-interleaver 220is received by the FEC decoder 222, which performs FEC decoding upon thereceived data bits. FEC decoder 222 produces recovered information bitsthat correspond exactly to the information bits absent errors introducedby the channel 212 and/or other components of the communication path.

FIG. 3 is a flow chart illustrating operation according to a firstembodiment of the present invention. The operation of FIG. 3 maps minput bits to 2^(m) modulation symbols of a two-dimensionalconstellation. The encoder 210 of FIG. 2, for example may employ thetwo-dimensional mappings determined according to the operation of FIG.3. Mapping of the m input bits to the 2^(m) modulation symbols commencesin forming a quarter-quadrant constellation of 2^(m−4) modulationsymbols that are located in a first-quadrant of the two-dimensionalsignal plane (step 302). The quarter-quadrant constellation formed has{fraction (1/16)}^(th) the size of the two-dimensional constellation.The illustrated embodiment of the present invention supports higherorder constellations, e.g., QAM constellations. The methodologyillustrated in FIG. 3 may be applied to a set of constellations so thatmapping of the constellations is consistent across the set ofconstellations. FIGS. 7A through 9B illustrate the manner in whichhigher-order constellations are formed according to the presentinvention, with particular reference to 256-QAM through 1024-QAMconstellations. Of course, the teachings of the present invention may beapplied to still larger constellations.

The method next includes uniquely associating each modulation symbol ofthe quarter-quadrant constellation with a respective m-4 bit label ofthe m input bits (step 304). A quarter constellation of thetwo-dimensional symbol constellation is then formed by adding to thequarter-quadrant constellation three copies of the quarter-quadrantconstellation (step 306). The three copies of the quarter-quadrantconstellation that are added are rotated by −90 degrees, 180 degrees,and −270 degrees, respectively. The quarter constellation is thenshifted by a shift value A such that the symbols coincide with thesymbols of the desired two-dimensional symbol constellation (step 308).When formed, the quarter constellation has ¼^(th) the size of thetwo-dimensional symbol constellation. The operations of step 308 may bemerged with the operations of step 306 in forming the quarterconstellation.

With the quarter constellation of the two-dimensional symbolconstellation formed, the method includes uniquely associating eachsymbol of the quarter constellation with a respective m-2 bit label ofthe m input bits (step 310). Of the m-2 bit label of the m input bits,m-4 bits of the m-2 bit label are inherited from the quarter-quadrantconstellation and two further bits of the m-2 bit label are used todistinguish quarter-quadrants of the quarter constellation.

Next, the method includes forming the two-dimensional symbolconstellation by adding to the quarter constellation (formed at step306-310), by adding to the quarter constellation three copies of thequarter constellation rotated by +90 degrees, 180 degrees, and +270degrees, respectively (step 312). Finally, with the two-dimensionalconstellation formed from the quarter constellations, the methodincludes uniquely associating each symbol of the two-dimensional symbolconstellations with an m-bit label (step 314). Of the m-bit label, m-2bits are inherited from the quarter constellation and two further bitsof the m input bits are used to distinguish quarters of thetwo-dimensional symbol constellation.

In one particular embodiment of step 310, of the two further bits of them-2 bit label used to distinguish quarter-quadrants of the quarterconstellation a value of [00] may correspond to a first quarter-quadrantof the quarter constellation having the quarter-quadrant constellation,a value of [01] corresponds to a second quarter-quadrant of the quarterconstellation having a quarter-quadrant constellation that has beenrotated by −90 degrees, a value of [11] corresponds to a thirdquarter-quadrant of the quarter constellation having a quarter-quadrantconstellation that has been rotated by 180 degrees, and a value of [10]corresponds to a fourth quarter-quadrant of the quarter constellationhaving a quarter-quadrant constellation that has been rotated by −270degrees.

In one particular embodiment of step 314, of the two further bits of them input bits used to distinguish quadrants of the two-dimensional symbolconstellation, a value of [00] corresponds to a first quadrant of thetwo-dimensional symbol constellation having the quarter constellation, avalue of [01] corresponds to a second quadrant of the two-dimensionalsymbol constellation having a quarter constellation that has beenrotated by −90 degrees, a value of [11] corresponds to a third quadrantof the two-dimensional symbol constellation having a quarterconstellation that has been rotated by 180 degrees, and a value of [10]corresponds to a fourth quadrant of the two-dimensional symbolconstellation having a quarter constellation that has been rotated by−270 degrees

FIG. 4 is a block diagram illustrating an encoder 210 constructedaccording to an embodiment of the present invention. The encoder 210includes trellis encoder processing 402 and a variable rate symbolmapper 404. One particular embodiment of the trellis encoder processing402 is shown in more detail in FIG. 11. The trellis encoder processing402 operates upon bits u⁰ to u⁹ to produce input bits y⁰ to y⁹. Notethat the illustrated number of bits operated upon by the symbol mapper404 is ten, which corresponds to a 1024-QAM constellation. When aservicing channel supports a smaller or larger constellation size, thenumber of bits operated upon will be lesser or greater, respectively.Because the quality of the servicing channel 212 (of FIG. 2) varies overtime, at any particular time, the symbol mapper 404 should use aconstellation size consistent with the quality of the servicing channel212.

The variable rate symbol mapper 404 maps input bits y⁰ to y⁹ totwo-dimensional symbol constellations according to the symbol mapping ofthe present invention (as was described with reference to FIG. 3).Symbol mapper 404 produces a two-dimensional symbol constellation as itsoutput. A constellation size selection input to the symbol mapper 404selects a constellation size for mapping of input bits y⁰ to y⁹ (or asubset or superset thereof) to a two-dimensional symbol constellation.

FIGS. 5A, 5B, 5C, and 6 are diagrams illustrating a generalized mappingof input bits to modulation symbols according to an embodiment of thepresent invention. FIG. 5A illustrates the formation of aquarter-quadrant constellation that includes a plurality of modulationsymbols. With m input bits mapped to a 2^(m) constellation according tothe present invention, the quarter-quadrant constellation includes2^(m−4) modulation symbols. The m input bits are denoted as y^(m−1),y^(m−2), . . . y^(s−1), y^(s), y^(s−1), . . . y³, y², y¹, y⁰ with y₀^(m−1)[y^(m−1), y^(m−2), . . . y^(s+1), y^(s), y^(s−1), . . . y³, y²,y¹, y⁰] being the symbol labels associated with the symbols of a2^(m)-QAM constellation. M-4 input bits of the m-4 input bits uniquelyassociate with the 2^(m−4) modulation symbols of the quarter-quadrantconstellation. The particular m-4 input bits of the m input bitsassociated with the 2^(m−4) modulation symbols of the quarter-quadrantconstellation depend upon the particular embodiment. FIGS. 7A and 7B,FIGS. 8A and 8B, and FIGS. 9A and 9B, illustrate particular embodimentsof quarter-quadrant mappings for 256-QAM, 512-QAM, and 1024-QAMconstellations, respectively.

Referring now to FIG. 5B, a quarter constellation of the two-dimensionalsymbol constellation is formed by adding to the quarter-quadrantconstellation three copies of the quarter-quadrant constellation. Thethree copies of the quarter-quadrant constellation added are rotated by−90 degrees, 180 degrees, and −270 degrees, respectively. The symbollabels of the four combined quarter-quadrant constellations areassociated with the label bits y^(s)y²=[00], [01], [1 1], [10] in theillustrated embodiment.

FIG. 5C illustrates the quarter constellation after it has been shiftedso that it corresponds to the symbols of the desired two-dimensionalsymbol constellation. In the embodiment of FIG. 5C, the quarterconstellation is shifted by −(1+j). This effectively shifts the quarterconstellation toward the center of the two-dimensional symbolconstellation.

FIG. 6 is a diagram illustrating a full constellation having input bitsmapped to modulation symbols according to the present invention. Asshown in FIG. 6, the full constellation is formed from copies of theshifted quarter constellation (of FIG. 5C) with additional bits of the minput bits used to distinguish quadrants of the full constellation. AFull 2 ^(m) QAM constellation is obtained by combining the quarterconstellation with three copies successively rotated by +90°. The symbollabels in the four rotated quarter constellations are associated withthe label bits y¹y⁰=[00], [01], [11], [10]. The symbols of the full2^(m) QAM constellation may be obtained algorithmically from the symbolsof the quarter-quadrant constellation by:a(y ₀ ^(m−1))=[a _(qq)(y ^(s+1) ^(m−1) ,y ₃ ^(s+1))×R ₁−(1+j)]×R₀,  Equation (1)

-   -   where R₁=j^(−2y) ^(s) ^(−(y) ^(⊕y) ²⁾ , R₀=j^(2y) ¹ ^(+(y) ¹        ^(⊕y) ⁰⁾ .

FIGS. 7A and 7B are diagrams illustrating aspects of the presentinvention as employed in forming a 256 quadrature amplitude modulated(QAM) constellation. As shown in FIG. 7A, a 256-QAM quarter-quadrantconstellation includes a plurality of modulation symbols each of whichis uniquely associated with a subset of the m input bits. For the256-QAM constellation illustrated, the m input bits include y⁷-y⁰ for atotal of 8-input bits. In such case, m-4 input bits y⁷, y⁶, y⁴, and y³uniquely associate with the modulation symbols of the quarter-quadrantconstellation. Such unique association is shown in particular in FIG.7A. For the 256-QAM constellation, the full two-dimensional symbolconstellation includes 256 unique modulation symbols. Thus, eachquadrant includes 64 modulation symbols and each quarter-quadrantincludes 16 modulation symbols. As is shown in FIG. 7B, the 256-QAM fullconstellation includes 256 modulation symbols each of which isassociated with the unique value of the 8 input bits.

To summarize the relationship between the m input bits of the full256-QAM constellation of FIG. 7B and the methodology of FIG. 3, the minput bits include eight bits [y⁷, y₆, y⁵, y⁴, y⁶, y², y¹, y⁰]. The m-4bits that uniquely associate with 2^(m−4) symbols of the quarter-quadrant constellation are bits [y⁷, y⁶, y⁴, y³]. The two further bitsof the m-2 bit label that distinguish quarter-quadrants are bits [y⁵,y²]. Finally, the two further bits of the m input bits that distinguishquadrants of the full constellation are bits [y¹, y⁰].

FIGS. 8A and 8B are diagrams illustrating aspects of the presentinvention when applied to a 512-QAM constellation. As contrasted to the256-QAM constellation of FIGS. 7A and 7B, the 512-QAM constellation ofFIG. 8B is not a square constellation but instead is a crossconstellation. Because the 512-QAM constellation is not square,formation of the constellation according to an embodiment of the presentinvention includes repositioning a plurality of quarter constellationsymbol positions when forming the quarter constellation. FIG. 8Aillustrates one example of such repositioning. Different embodiments ofthe present invention may employ differing manners of repositioning theplurality of quarter constellation symbols.

To summarize the relationship between the m input bits of the full512-QAM constellation of FIG. 8B and the methodology of FIG. 3, the minput bits include nine bits [y⁸, y⁷, y⁶, y⁵, y⁴, y³, y², y¹, y⁰]. Them-4 bits that uniquely associate with the 2 m-4 symbols of thequarter-quadrant constellation are bits [y⁸, y⁷, y⁶, y⁴, y³]. The twofurther bits of the m-2 bit label that distinguish quarter-quadrants arebits [y⁵, y²] Finally, the two further bits of the m input bits thatdistinguish quadrants of the full constellation are bits [y¹, y⁰].

FIGS. 9A and 9B are diagrams illustrating aspects of the presentinvention as applied to a 1024-QAM constellation. FIG. 9A illustratesthe unique association between modulation symbols of thequarter-quadrant constellation with the respective m-4 bit label of them input bits. With the 1024-QAM quarter-quadrant constellation, the minput bits include ten bits [y⁹, y⁸, y⁷, y⁶, y⁵, y⁴, y³, y², y¹, y⁰].The m-4 bits that uniquely associate with the 2^(m−4) symbols of thequarter-quadrant constellation are bits [y⁹, y⁸, y⁷, y⁵, y⁴, y³]. Thetwo further bits of the m-2 bit label that distinguish quarter-quadrantsare bits [y⁶, y²]. Finally, the two further bits of the m input bitsthat distinguish quadrants of the full constellation are bits [y¹, y⁰].

FIG. 10 is a flow chart illustrating operation according to the presentinvention in creating input bits from information bits according to anembodiment of the present invention. The operation of FIG. 10 is simplyone example of a manner in which the plurality of input bits may beconstructed. The operations of FIG. 10 are substantially consistent withthe standardized operations of the J.83 annex to DOCSIS. The operationsof FIG. 10 are also consistent with the structure of FIG. 2 and theoperations of FIG. 3.

Operation commences at step 1002 wherein FEC coding is performed on theplurality of information bits to produce a plurality of FEC blocks. Aswill be described further with reference to FIGS. 11 and 12 the FECcoding operation depends upon the constellation size employed. In oneparticular embodiment, Reed-Solomon encoding is employed to operate uponthe information bits to produce the FEC blocks. With the FEC blocksformed, the FEC blocks are then interleaved at step 1004 and randomizedat step 1006. Then, a frame synch trailer is appended to the pluralityof interleaved and randomized FEC blocks to form an FEC frame (step1008). The FEC frame has an integer number of trellis groups so that theFEC frame may be trellis encoded. The plurality of trellis groups of theFEC frame are then trellis encoded and mapped to a plurality oftwo-dimensional symbols (step 1010).

FIG. 11 is a block diagram illustrating an encoder that employs trellisencoding operations according to an aspect of the present invention. Asshown in FIG. 11, trellis encoder processing 402 includes a differentialprecoder and a block convolutional coder (BCC) that produce trellis bitsy⁰ and y¹. These trellis bits are employed to select a quarterconstellation of a two-dimensional symbol constellation constructedaccording to the present invention. In the example of FIG. 11, fivetrellis groups are operated upon at one time to produce 5-QAM symbols.The size of the trellis groups is directly related to the size of theQAM symbols employed in the particular operation. As was previouslydescribed, the size of the constellation employed depends upon channelconditions and may depend upon the capabilities of receiving device.

FIG. 12 is a diagram illustrating the manner in which forward errorcorrection frames may be constructed according to various embodiments ofthe present invention. With the embodiments of FIG. 12, the FEC framestructures depend upon the constellation size of a two-dimensionalsymbol constellation employed. FEC frame structures are chosen such thatall frames comprise an integer number of Reed-Solomon code words,comprise an integer number of trellis groups, have similar temporallength, and allow for at least 40 Frame Synch Trailer (FST) bits.256-QAM FEC frames may be constructed according to annex J.83B ofDOCSIS, for example. The manner in which other FEC frames areconstructed are examples of particular embodiments and vary fromembodiment to embodiment.

Referring now to FIGS. 2, 10, 11, and 12, a 256-QAM FEC frame includes88 RS code words that have been interleaved and scrambled and appendedwith 40 Frame Synch Trailer (FST) bits. The FST bits are inserted “asthe last (u¹, u⁰) bits” within a FEC frame. Thus, the length of a FECframe is 78'888 bits. The 78'888 bits of a FEC frame are divided into2076 blocks of 38 bits, which are referred to as trellis groups (TGs).The trellis encoder 210 converts the sequence of TGs into blocks of five256 QAM symbols per input TG. Trellis coding therefore occurs at a coderate of 38/5×8=0.95.

The 38 bits D₀, D₁, . . . D₃₇ of one TG are arranged prior to trellisencoding and modulation as depicted in FIG. 11. Trellis encoding isaccomplished by the Differential Precoder and two parallel rate R=4/5Binary Convolutional enCoders (BCCs), whose functions are detailed inthe J.83B annex to DOCSIS. During one TG period, bits D₁, D₉, D₁₇, D₂₅enter sequentially the differential encoder as bits u¹, while bits D₀,D₈, D₁₆, D₂₄ enter sequentially the differential precoder as bits u⁰.The differential precoder generates during one TG period four bits v¹and four bits v⁰. The four bits v¹ are encoded by the first BCC into 5coded bits Y¹ ₀, Y¹ ₁, Y¹ ₂, Y¹ ₃, Y¹ ₄. Similarly, the four bits v⁰bits are encoded by the second BCC into 5 coded bits Y⁰ ₀, Y⁰ ₁, Y⁰ ₂,Y⁰ ₃, Y⁰ ₄.

The 256-QAM symbol mapper translates symbol labels y₀ ⁷

[y⁷, y⁶, y⁵, y₄, y³, y², y¹, y⁰] into 256 QAM symbols. During one TGperiod, five 256 QAM symbols are generated. The label of the firstsymbol is y₀ ⁷

[D₇,D₆,D₅,D₄,D₃,D₂, Y¹ ₀, Y⁰ ₀]; the label of the second symbol is y₀ ⁷

[D₁₅,D₁₄,D₁₃, D₁₂,D₁₁, D₁₀, Y¹ ₁, Y⁰ ₁]; etc.; the label of the fifthsymbol is [D₃₇, D₃₆, D₃₅, D₃₄, D₃₃, D₃₂, Y¹ ₄, Y⁰ ₄]. The 256-QAM symbolmapping is given in the J.83B annex to DOCSIS in the form of a 16×1 6array of cells, each containing an 8-bit label [y⁷, y⁶, y⁵, y⁴, y², y¹,y⁰] in plain numerical form.

FIG. 12 also illustrates the manner in which TGs may be formed for128-QAM constellations, 256-QAM constellations, 512-QAM constellations,and 1024-QAM constellations. For the 128-QAM constellation, each 33-bitTG includes bits: D₀,D₁,D₂,D₃,D₄,D₅,D₆,D₇,D₈,D₉,D₁₀,D₁₁,D₁₂,D₁₃,D₁₄,D₁₅,D₁₆,D₁₇,D₁₈,D₁₉,D₂₀,D₂₁,D₂₂,D₂₃,D₂₄,D₂₅,D₂₆,D₂₇,D₂₈,D₂₉,D₃₀,D₃₁,D₃₂=D_(last),wherein the bolded bits serve as inputs to the Trellis encoderprocessing 402. For the 256-QAM constellation, each 38-bit TG includesbits: D₀,D₁,D₂,D₃,D₄,D₅,D₆,D₇,D₈,D₉,D₁₀,D₁₁,D₁₂,D₁₃,D₁₄,D₁₅,D₁₆,D₁₇,D₁₈,D₁₉,D₂₀,D₂₁,D₂₂,D₂₃,D₂₄,D₂₅,D₂₆,D₂₇,D₂₈,D₂₉,D₃₀,D₃₁D₃₂,D₃₃ D₃₄,D₃₅ D₃₆,D₃₇=D_(last), wherein the bolded bits serve asinputs to the Trellis encoder processing 402. For the 512-QAMconstellation, each 43-bit TG includes bits:D₀,D₁,D₂,D₃,D₄,D₅,D₆,D₇,D₈,D₉,D₁₀,D₁₁,D₁₂,D₁₃,D₁₄,D₁₅,D₁₆,D₁₇,D₁₈,D₁₉,D₂₀,D₂₁,D₂₂,D₂₃,D₂₄,D₂₅,D₂₆,D₂₇,D₂₈,D₂₉,D₃₀,D₃₁,D₃₂,D₃₃,D₃₄,D₃₅ . . . , D₄₂=D_(last), wherein the bolded bits serve asinputs to the Trellis encoder processing 402. Finally, for the 1024-QAMconstellation, each 48 bit TG includes bits:D₀,D₁,D₂,D₃,D₄,D₅,D₆,D₇,D₈,D₉,D₁₀,D₁₁,D₁₂,D₃,D₁₄,D₁₅,D₁₆,D₁₇,D₁₈,D₁₉,D₂₀,D₂₁,D₂₂,D₂₃,D₂₄,D₂₅,D₂₆,D₂₇,D₂₈,D₂₉,D₃₀,D₃₁,D₃₂,D₃₃,D₃₄,D₃₅ . . . , D₄₇=D_(last), wherein the bolded bitsserve as inputs to the Trellis encoder processing 402.

The invention disclosed herein is susceptible to various modificationsand alternative forms. Specific embodiments therefore have been shown byway of example in the drawings and detailed description. It should beunderstood, however, that the drawings and detailed description theretoare not intended to limit the invention to the particular formdisclosed, but on the contrary, the invention is to cover allmodifications, equivalents and alternatives falling within the spiritand scope of the present invention as defined by the claims.

1. A method for mapping m input bits to 2^(m) modulation symbols of atwo-dimensional symbol constellation, the method comprising: forming aquarter-quadrant constellation of 2^(m−4) modulation symbols that arelocated in a first quadrant of the two-dimensional signal plane, thequarter-quadrant constellation having one-sixteenth the size of thetwo-dimensional symbol constellation; uniquely associating eachmodulation symbol of the quarter-quadrant constellation with arespective m-4 bit label of the m input bits; forming a quarterconstellation of the two-dimensional symbol constellation by adding tothe quarter-quadrant constellation three copies of the quarter-quadrantconstellation rotated by −90 degrees, 180 degrees, and −270 degrees,respectively, and then displacing the quarter constellation by a shiftvalue A such that the symbols coincide with symbols of the desiredtwo-dimensional symbol constellation, wherein the quarter constellationhas one-fourth the size of the two-dimensional symbol constellation;uniquely associating each symbol of the quarter constellation with arespective m-2 bit label of the m input bits, wherein m-4 bits of them-2 bit label are inherited from the quarter-quadrant constellation andtwo further bits of the m-2 bit label are used to distinguishquarter-quadrants of the quarter constellation; forming thetwo-dimensional symbol constellation by adding to the quarterconstellation three copies of the quarter constellation rotated by +90degrees, 180 degrees, and +270 degrees, respectively; and uniquelyassociating each symbol of the two-dimensional symbol constellation withan m-bit label, wherein m-2 bits of the m-bit label are inherited fromthe quarter constellation and two further bits of the m input bits areused to distinguish quarters of the two-dimensional symbolconstellation.
 2. The method of claim 1, wherein of the two further bitsof the m-2 bit label used to distinguish quarter-quadrants of thequarter constellation: a value of [00] corresponds to a firstquarter-quadrant of the quarter constellation having thequarter-quadrant constellation; a value of [01] corresponds to a secondquarter-quadrant of the quarter constellation having a quarter-quadrantconstellation that has been rotated by −90 degrees; a value of [11]corresponds to a third quarter-quadrant of the quarter constellationhaving a quarter-quadrant constellation that has been rotated by 180degrees; and a value of [10] corresponds to a fourth quarter-quadrant ofthe quarter constellation having a quarter-quadrant constellation thathas been rotated by −270 degrees.
 3. The method of claim 1, wherein ofthe two further bits of the m input bits used to distinguish quadrantsof the two-dimensional symbol constellation: a value of [00] correspondsto a first quadrant of the two-dimensional symbol constellation havingthe quarter constellation; a value of [01] corresponds to a secondquadrant of the two-dimensional symbol constellation having a quarterconstellation that has been rotated by −90 degrees; a value of [11]corresponds to a third quadrant of the two-dimensional symbolconstellation having a quarter constellation that has been rotated by180 degrees; and a value of [10] corresponds to a fourth quadrant of thetwo-dimensional symbol constellation having a quarter constellation thathas been rotated by −270 degrees.
 4. The method of claim 1, wherein: thetwo-dimensional symbol constellation is a 256-QAM constellation; the minput bits comprise eight bits [y⁷, y⁶, y⁵, y⁴, y³ y², y¹, y⁰]; the2^(m−4) bits are bits [y⁷ y⁶, y⁴ y³]; the two further bits of the m-2bit label are bits [y⁵, y²]; and the two further bits of the m inputbits are bits [y¹, y⁰].
 5. The method of claim 1, wherein: thetwo-dimensional symbol constellation is a 1024-QAM constellation; the minput bits comprise ten bits [y⁹,y⁸, y⁷, y⁵, y⁴, y³, y², y¹, y⁰]; the2^(m−4) bits are bits [y⁹, y⁸, y⁷, y⁵, y⁴, y³]; the two further bits ofthe m-2 bit label are bits [y⁶, y²]; and the two further bits of the minput bits are bits [y¹, y⁰].
 6. The method of claim 1, wherein formingthe quarter constellation further comprises repositioning a plurality ofquarter constellation symbol positions.
 7. The method of claim 6,wherein: the two-dimensional symbol constellation is a 128 QAMconstellation; the m input bits comprise seven bits [y⁶, y⁵, y⁴, y³, y²,y¹, y⁰]; the 2^(m−4) bits are bits [y⁶, y⁴, y³]; the two further bits ofthe m-2 bit label are bits [y⁵, y²]; and the two further bits of the minput bits are bits [y¹, y⁰].
 8. The method of claim 6, wherein: thetwo-dimensional symbol constellation is a 512-QAM constellation; the minput bits comprise eight bits [y⁸, y⁷, y⁶,y⁵,y⁴,y³,y², y¹, y⁰]; the2^(m−4) bits are bits [y⁸, y⁷, y⁵, y⁴, y³]; the two further bits of them-2 bit label are bits [y⁶, y²]; and the two further bits of the m inputbits are bits [y¹, y⁰].
 9. The method of claim 1, further comprisingforming the m input bits from a plurality of information bits byoperating upon the plurality of information bits to form a Forward ErrorCorrection (FEC) frame.
 10. The method of claim 9, wherein the FEC frameincludes an integer number of trellis groups.
 11. The method of claim 9,wherein Reed-Solomon encoding is employed to form the FEC frame.
 12. Themethod of claim 1, further comprising forming the m input bits from aplurality of information bits by: Forward Error Correction (FEC) codingthe plurality of information bits to produce a plurality of FEC blocks;interleaving and randomizing the plurality of FEC blocks; and appendinga frame synch trailer to the plurality of interleaved and randomized FECblocks to form a FEC frame.
 13. The method of claim 9, wherein the FECframe includes an integer number of trellis groups.
 14. A method formapping m input bits to a two-dimensional symbol constellation, themethod comprising: forming a quarter-quadrant constellation of 2^(m−4)modulation symbols; forming a quarter constellation of thetwo-dimensional symbol constellation by adding to the quarter-quadrantconstellation three copies of the quarter-quadrant constellation rotatedby −90 degrees, 180 degrees, and −270 degrees, respectively, and thendisplacing the constellation by a shift value Δ such that the symbolscoincide with symbols of the desired two-dimensional symbolconstellation, wherein the quarter constellation has one-fourth the sizeof the two-dimensional symbol constellation; and forming thetwo-dimensional symbol constellation by adding to the quarterconstellation three copies of the quarter constellation rotated by +90degrees, 180 degrees, and +270 degrees, respectively.
 15. The method ofclaim 14, wherein forming the quarter-quadrant constellation furthercomprises uniquely associating each symbol of the quarter-quadrantconstellation with a respective m-4 bit label of the m input bits. 16.The method of claim 14, wherein forming the quarter constellationfurther comprises uniquely associating each symbol of the quarterconstellation with a respective m-2 bit label of the m input bits,wherein m-4 bits of the m-2 bit label are inherited from the symbols ofthe quarter-quadrant constellation and two further bits of the m-2 bitlabel are used to distinguish quarter-quadrants of the quarterconstellation.
 17. The method of claim 16, wherein of the two furtherbits of the m-2 bit label used to distinguish quarter-quadrants of thequarter constellation: a value of [00] corresponds to a quarter-quadrantof the quarter constellation having an unrotated quarter-quadrantconstellation; a value of [01] corresponds to a second quarter-quadrantof the quarter constellation having a quarter-quadrant constellationthat has been rotated by −90 degrees; a value of [11] corresponds to athird quarter-quadrant of the quarter constellation having aquarter-quadrant constellation that has been rotated by 180 degrees; anda value of [10] corresponds to a fourth quarter-quadrant of the quarterconstellation having a quarter-quadrant constellation that-has beenrotated by −270 degrees.
 18. The method of claim 14, wherein forming thetwo-dimensional symbol constellation further comprises uniquelyassociating each symbol of the two-dimensional symbol constellation withan m-bit label, wherein m-2 bits of the m-bit label are inherited fromthe quarter constellation and two further bits of the m input bits areused to distinguish quadrants of the two-dimensional symbolconstellation.
 19. The method of claim 18, wherein of the two furtherbits of the m input bits used to distinguish quadrants of thetwo-dimensional symbol constellation: a value of [00] corresponds to afirst quadrant of the two-dimensional symbol constellation having thequarter constellation; a value of [01] corresponds to a second quadrantof the two-dimensional symbol constellation having a quarterconstellation that has been rotated by −90 degrees; a value of [11]corresponds to a third quadrant of the two-dimensional symbolconstellation having a quarter constellation that has been rotated by180 degrees; and a value of [10] corresponds to a fourth quadrant of thetwo-dimensional symbol constellation having a quarter constellation thathas been rotated by −270 degrees.
 20. The method of claim 14, furthercomprising forming the m input bits from a plurality of information bitsby operating upon the plurality of information bits to form a ForwardError Correction (FEC) frame.
 21. The method of claim 20, wherein theFEC frame includes an integer number of trellis groups.
 22. The methodof claim 20, wherein Reed-Solomon encoding is employed to form the FECframe.
 23. The method of claim 14, further comprising forming the minput bits from a plurality of information bits by: Forward ErrorCorrection (FEC) coding the plurality of information bits to produce aplurality of FEC blocks; interleaving and randomizing the plurality ofFEC blocks; and appending a frame synch trailer to the plurality ofinterleaved and randomized FEC blocks to form a FEC frame.
 24. Themethod of claim 23, wherein the FEC frame includes an integer number oftrellis groups.
 25. A transmitter comprising: Forward Error Correctionblock that receives a plurality of information bits and that produces aplurality of FEC blocks; an interleaver that interleaves the pluralityof FEC blocks; a randomizer that randomizes the plurality of FEC blocks;a frame synch trailer block that appends a frame synch trailer block tothe plurality of interleaved and randomized FEC blocks to form an FECframe having a plurality of input bits; and an encoder that maps m inputbits of the plurality of input bits to a two-dimensional symbolconstellation that that was constructed by: forming a quarter-quadrantconstellation of 2^(m−4) modulation symbols; forming a quarterconstellation of the two-dimensional symbol constellation by adding tothe quarter-quadrant constellation three copies of the quarter-quadrantconstellation rotated by −90 degrees, 180 degrees, and −270 degrees,respectively, and then displacing the constellation by a shift value Asuch that the symbols coincide with symbols of the desiredtwo-dimensional symbol constellation, wherein the quarter constellationhas one-fourth the size of the two-dimensional symbol constellation; andforming the two-dimensional symbol constellation by adding to thequarter constellation three copies of the quarter constellation rotatedby +90 degrees, 180 degrees, and +270 degrees, respectively.
 26. Anapparatus for mapping m input bits to 2^(m) modulation symbols of atwo-dimensional symbol constellation, the apparatus comprising: meansfor forming a quarter-quadrant constellation of 2^(m−4) modulationsymbols that are located in a first quadrant of the two-dimensionalsignal plane, the quarter-quadrant constellation having one-sixteenththe size of the two-dimensional symbol constellation; means for uniquelyassociating each modulation symbol of the quarter-quadrant constellationwith a respective m-4 bit label of the m input bits; means for forming aquarter constellation of the two-dimensional symbol constellation byadding to the quarter-quadrant constellation three copies of thequarter-quadrant constellation rotated by −90 degrees, 180 degrees, and−270 degrees, respectively, and then displacing the constellation by ashift value A such that the symbols coincide with symbols of the desiredtwo-dimensional symbol constellation, wherein the quarter constellationhas one-fourth the size of the two-dimensional symbol constellation;means for uniquely associating each symbol of the quarter constellationwith a respective m-2 bit label of the m input bits, wherein m-4 bits ofthe m-2 bit label are inherited from the quarter-quadrant constellationand two further bits of the m-2 bit label are used to distinguishquarter-quadrants of the quarter constellation; means for forming thetwo-dimensional symbol constellation by adding to the quarterconstellation three copies of the quarter constellation rotated by +90degrees, 180 degrees, and +270 degrees, respectively; and means foruniquely associating each symbol of the two-dimensional symbolconstellation with an m-bit label, wherein m-2 bits of the m-bit labelare inherited from the quarter constellation and two further bits of them input bits are used to distinguish quarters of the two-dimensionalsymbol constellation.